In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are a good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
In prior art various materials have been proposed for use as OSCs in OFETs, including small molecules like for example pentacene, and polymers like for example polyhexylthiophene. However, the materials and devices investigated so far do still have several drawbacks, and their properties, especially the processability, charge-carrier mobility, on/off ratio and stability do still leave room for further improvement.
There is especially a strong need for novel p-type organic semiconductor for application in bulk heterojunction photovoltaics that can yield improved device performance. The limitations of existing p-type materials relate to deficiencies in light absorption, oxidative stability and charge-carrier mobility.
State-of-the-art p-type materials for bulk heterojunction photovoltaics include poly(3-hexylthiophene) (P3HT) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) as shown below:

P3HT is strongly limited by the onset of its absorption profile (˜650 nm) that limits its ability to absorb light from the solar spectrum. Furthermore, the energy of P3HT's HOMO energy level renders it susceptible to oxidative doping. PCPDTBT has an improved absorption profile compared to P3HT, with an absorption onset of 890 nm [see Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Muhlbacher, M. Scharber and C. Brabec, Macromolecules, 2007, 40, 1981] however it suffers from a lower intrinsic mobility, which was reported to be in the region of 10−2 cm2/Vs [see above citation] and a poorer morphology when blended with the commonly used n-type material PCBM, [see D. Muhlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana and C. Brabec, Adv. Mater., 2006, 18, 2884] in comparison to optimised P3HT:PCBM blends.
Therefore, there is still a need for improved p-type organic semiconductors, especially for use in bulk heterojunction OPV devices, which do not have the drawbacks of the materials of prior art. In particular, the new materials should demonstrate the following properties:                low bandgap,        high charge carrier mobility,        being easy to synthesize,        high solubility in organic solvents,        good processability for the device manufacture process,        high oxidative stability,        long lifetime in electronic devices.        
One aim of the present invention is to provide new p-type OSC materials, especially for use in bulk heterojunction OPV devices, fulfilling the above-mentioned requirements. Another aim is to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing materials as described hereinafter, which represent a novel class of conjugated polymer based upon the bis(thienocyclpenta) benzothiadiazole unit and derivatives thereof.